It is doubtful that the current taxonomy provides a true picture of the diversity of the genus Aotus. Ruiz-Herrera et al. (2005) reported that cytogenetic studies have characterized 18 different karyotypes with diploid numbers ranging from 46 to 58 chromosomes. The taxonomy of the night monkeys essentially follows the revision by Hershkovitz (1983), with some modifications for the Colombian and Central American forms.

Reviewing the entire taxonomy and distributions of the night monkeys, Aotus, Ford (1994) carried out multivariate analyses of craniodental measures and pelage patterns and color, and also took into consideration chromosomal data and blood protein variations. Ford (1994) concluded that there was “good support” for just two species north of the Río Amazonas: A. trivirgatus (Humboldt, 1812) east and north of the Rio Negro, and the polymorphic A. vociferans to the west of the Rio Negro. Aotus vociferans, as such, would include all the forms north of the Río Amazonas/Solimões in Brazil (west of the Rio Negro), Peru, Colombia and Ecuador, and in the Chocó, northern Colombia and Colombian Andes, and Panama: brumbacki, lemurinus, griseimembra, and zonalis.

The karyotype of Aotus trivirgatus is not known (Defler and Bueno 2007), and Hershkovitz (1983), who based his taxonomy primarily on karyotypes, indicated that further research may result in a revision of this and the forms he recognizes as miconax and infulatus.

Justification:
This species is listed as Least Concern due to its wide range, presumed large populations, and because there are no major threats believed to be resulting in a significant population decline at present.

Based on a specimen collected at Maipures, near the mouth of the Río Tomo on the Río Orinoco, Defler and Bueno (2007) indicated that the night monkeys of eastern central Colombia north of the Río Guaviare are probably A. brumbacki.

Hershkovitz (1983) proposed that A. trivirgatus was the form in Colombia between the Rios Vaupés and Inirida, but Defler (2003, 2004) indicated that A. vociferans occurred in this region and that A. trivirgatus as such is restricted to the east of the Ríos Negro, lower Guainia, and Atabapo and, below the mouth the Río Atabapo, the Río Orinoco. To the north, in Venezuela, A. trivirgatus occurs south of the Río Orinoco and east as far the middle Rio Caroni. The extension east beyond the Río Caura is based on just one locality on the upper Caroni (Bodini and Pérez-Hernandez 1987; Linares 1998). A. trivrigatus is confined to north (left bank) of the Rio Negro to its mouth, extending east, north of the Rio Amazonas, as far as the Rio Trombetas. North it reaches the Serra da Pacaraima, but does not it seem extend into Guyana. In the state of Roraima, the Rio Mucajaí is indicated (by Hershkovitz (1983) as marking its northern limits.

Night monkeys are nocturnal: they are most active at dawn and dusk. The only exception is Aotus azarae azarae of the Chaco of southern Bolivia, Paraguay and Argentina, which is cathemeral (active during night and day). They are frugivorous; their diet includes fruit, nectar and flower (seasonally important for the A. a. azarae in the Chaco), leaves, and small animals prey such as insects (Wright 1989; Fernandez-Duque 2007).

They are socially monogamous, living in small groups of an adult pair and offspring of different ages (infant, one or two juveniles and sometimes a subadult. In A. a. azarae a significant number of adults range alone. They may be subadults that have left their natal groups or older adults which have been evicted from their groups by competitors (Fernandez-Duque and Huntington 2002; Fernandez-Duque 2004). Both sexes disperse. Males care for the infants (carry them) (Rotundo et al. 2002, 2005). Lone adults were observed by Villavicencio Galindo (2003) in northern Colombia. Night monkeys are territorial—groups occupy overlapping territories of 5-18 ha (depending on the species and location) (Wright 1978, 1981; Fernandez-Duque 2007). Wright (1994) and Fernandez-Duque (2007) review the behaviour and ecology of the genus.

Captive male A. lemurinus reach sexual maturity when 2 years old, and captive female A. vociferans and A. nancymaae first breed when 3-4 years old (Dixson 1983; Fernandez-Duque 2007). In the wild, male A. azarae reach adult weight only when about 4 years old, and age at first reproduction is about 5 years of age (Juárez et al. 2003; Fernandez-Duque 2004). Single offspring are the rule. Wright (1985) recorded births between August and February for A. nigriceps in Peru (Manu National Park), and Aquino et al. (1990) indicated a birth season between December and March for A. nancymaae in north-eastern Peru.